JP2834916B2 - Inverter microwave oven drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive circuit for an inverter microwave oven which converts a low-voltage DC power supply into a high-voltage high-frequency power and supplies power to a magnetron.

[0002]

2. Description of the Related Art In recent years, various devices have been developed in consideration of the outdoor use of electric and electronic devices which have been used with a commercial AC power supply, and an inverter microwave oven widely used in homes at present. Are also being used outdoors.

FIG. 8 shows a circuit configuration of a conventional general inverter microwave oven. In this circuit, a commercial power supply (AC100
V, 50/60 Hz) is first converted to DC power by a rectifier circuit. This DC power is converted to a high frequency by a single-stone resonance type inverter circuit, and is boosted to several kV by a boosting transformer. The transformer output is rectified by the voltage doubler rectifier circuit and supplies power to the magnetron.

On the other hand, when the inverter microwave oven is used outdoors, it is necessary to use a limited energy source such as a DC 12 V or 24 V storage battery. For example, as shown in FIG. A DC / AC inverter is provided between a low-voltage DC power supply such as that described above and a circuit used in an inverter microwave oven that uses the AC 100 V as a power supply, and the DC / AC inverter converts DC low voltage to AC 100 V. , Had driven the microwave oven.

[0005]

When an inverter microwave oven is used by using a storage battery or the like, the method of inserting a DC / AC inverter between the DC / AC inverter and the AC / DC converter requires two conversions. Therefore, the power utilization rate decreases. In addition, the cost is increased by adding the DC / AC inverter.

Therefore, an inverter circuit for directly driving a magnetron using a low-voltage DC power supply such as a storage battery is required. To satisfy this demand, the present applicant has developed an inverter microwave oven suitable for a low-voltage DC power supply. Driving circuits have already been proposed (Japanese Patent Application Nos. 2-200689 and 2-2).
00690).

By the way, in a conventional inverter circuit, a PWM method for controlling output power by controlling a duty ratio of an on / off time of a switching element is generally used. However, the above method has a disadvantage that switching loss increases because there is a period in which both the current and the voltage change sharply when the switching element is turned on and off. As a means for solving this drawback, there is a method using a series resonance circuit composed of a capacitor and a coil.
In this case, the current waveform when the switching element is turned on becomes a sine wave by the series resonance circuit, and the current or voltage crosses at almost zero when the switching element is turned on and off. Therefore, switching loss can be significantly reduced.
However, while the resonance frequency of the series resonance type inverter is uniquely determined, the PWM control changes the duty ratio to control the output power.
It is difficult to change the output power without impairing the above characteristics. In order to solve this problem, there is a method of controlling the frequency by changing the frequency. However, it is necessary to design the transformer at the lowest frequency at which it operates. The present invention has been made in view of the above,
The purpose is to drive the inverter microwave oven as previously proposed (Japanese Patent Application Nos. 2-200689 and 2968).
To provide a low-cost, compact, high-output, and high-efficiency inverter microwave drive circuit using a low-voltage DC power supply as a power supply, and a drive circuit that can easily perform output power control. Is to provide.

[0008]

A drive circuit for an inverter microwave oven according to the present invention is a drive circuit for an inverter microwave oven that directly converts a power supply into a high-frequency current, and has one switching element or two switching elements connected in parallel. Sui more
An inverter circuit including two switching element groups each including a switching element, and control means for performing on / off operations of the switching element; and a high-frequency alternating current supplied from the inverter circuit to a primary winding and a secondary winding. Transformer for generating a high voltage to the motor and a secondary winding of the boost transformer.
And consisted of a high voltage diode and a high voltage capacitor.
A voltage doubler rectifier circuit for supplying power to the magnetron, a low-voltage capacitor connected in parallel to the power supply, and a choke coil connected to one end of the low-voltage capacitor and connected in series to the power supply. A primary winding and the two switching element groups are connected to form a closed loop, and a connection point between the two switching element groups and a center tap of the primary winding of the step-up transformer are connected to both ends of the low-voltage capacitor. It is characterized by having been done.

Further, the control means in the inverter circuit may determine that the on-time is substantially half of the oscillation period with respect to the relationship between the oscillation period of the transient oscillation of the output current waveform of the inverter circuit and the on-time of the switching element. It is characterized in that it is set so as to fall between one cycle and one cycle.

Furthermore, the parallel-connected low-pressure condenser to the power supply having two or more inserts into a circuit connected to the low pressure condenser, a switching means for disconnecting said controlling said switching means It is characterized in that output power control is performed without changing the timing of zero current switching by changing the capacitance of the low voltage capacitor. Furthermore, a choke coil connected in series to the power supply
Having two or more coils and connected to the choke coil
Switching means for insertion and disconnection from the road
And controlling said switching means to control said choke coil.
Zero current switching by changing the reactor
Output power control without breaking the timing of
Features.

[0011]

[Operation] By the inverter circuit, two switching elements
One of the switching elements
When the child group is turned on, the high voltage of the secondary side circuit of the step-up transformer
Current flows so that the capacitor is charged, then the same switch
When the switching element group is turned off,
While the supplied electromagnetic energy is supplied to the high-voltage capacitor,
Regenerated by the source.

Next, the switch circuit is switched by the inverter circuit.
When the other of the switching elements is turned on, the secondary side of the step-up transformer
Electric current flows through the magnetron of the circuit, and electricity is supplied to the magnetron.
Energy is supplied. Where it is fed to the magnetron
Power is the voltage of the high-voltage capacitor and the switching elements.
Determined by the length of on-time. Next, the same switching element group
Is turned off, the electromagnetic energy stored in the step-up transformer
Is regenerated to a power supply while being supplied to a high-voltage capacitor.
The above operation is repeated, and the magnetron
Continue to oscillate. Here, the circuit in
Switching elements that draw an arc of vibration determined by impedance
Is charged with a current waveform similar to the current waveform flowing through
An arc of vibration determined by the circuit impedance also for Gnetron
With a current waveform similar to the current waveform of the switching element group
Electrical energy is supplied. Where the circuit impedance
Is the leakage inductance of the step-up transformer in the circuit.
The capacitance of the high voltage capacitor and the low voltage capacitor.
Inductor determined by the sensor, low-voltage choke coil, and circuit resistance
It is determined by the capacitance, capacitance and resistance.

The current waveform of the switching element oscillates at a natural frequency determined by a circuit constant, that is, an inductance component, a capacitance component, and a circuit resistance in the main circuit. By adjusting the inductance value, the capacitance value, or both of them, If the ON time of the switching element is set within a period from one half period to one period of the natural frequency, zero current switching can be performed, the transition loss becomes very small, and the switching loss can be reduced.

When the capacitance value of the low-voltage capacitor is further changed from this state, the average value of the output current changes due to a change in the waveform shape due to a change in the oscillation condition of the output current waveform without breaking the zero current switching state. However, since the output power is proportional to the average output current, the output power of the inverter is controlled.

[0015]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the configuration of a drive circuit of an inverter microwave oven according to an embodiment of the present invention;

FIG. 1 is a power supply circuit diagram of a first embodiment of the present invention. The power supply circuit includes a push-pull voltage type inverter circuit 2 for converting DC power of a low-voltage DC power supply (for example, a storage battery for an automobile) into high-frequency power, a boosting transformer 3 for boosting a power supply voltage, and rectifying an output of the boosting transformer 3. The magnetron 5 is driven by an output of the voltage doubler half-wave rectifier 4. Further, a power supply for heating the filament of the magnetron 5 is also supplied from the secondary side of the step-up transformer 3.

The voltage doubler half-wave rectifier circuit 4 includes two high voltage diodes 6a and 6b and a high voltage capacitor 7.

The inverter circuit 2 is a power MOSFET for switching one or more DCs at high speed.
(Metal Oxide Semi-conductor Field Effect Transist
or abbreviations), two switching element groups 8 connected in parallel
a, 8b, switching element drive circuits 9a, 9b for driving switching element groups 8a, 8b, and control circuit 1
0.

The drains of the power MOSFETs forming the switching element groups 8a and 8b are connected to one end 3a and the other end 3b of the primary winding of the step-up transformer 3, respectively, to form these two switching element groups 8a and 8b. The sources of the power MOSFETs are connected to each other, and the power M that forms the switching element groups 8a and 8b
OSFET gate is switching element drive circuit 9
a, 9b.

The switching element groups 8a and 8b composed of power MOSFETs are driven by the control circuit 10 and the switching element drive circuits 9a and 9b, and the current flowing on the primary side of the step-up transformer 3 is switched at high speed.

The DC power supply 1 has a source connected to a switching element group 8a and a switching element group 8b via a choke coil 11 having one end connected in series and a low-voltage capacitor 12 connected in parallel to the DC power supply 1 and the choke coil 11.
And the other end is connected to the step-up transformer 3
Is connected to the center tap 3c of the primary winding.

Next, the operation of this embodiment will be described. When the power MOSFETs of the switching element group 8b are turned on from the state where both the power MOSFETs of the switching element groups 8a and 8b are off, the secondary circuit of the boosting transformer 3 includes the high-voltage capacitor 7, the high-voltage diode 6a, and the boosting transformer 3 A current flows through a closed loop of one end 3e of the secondary winding 3e and the other end 3d of the secondary winding, and the high-voltage capacitor 7 is charged.

Next, when the power MOSFET of the switching element group 8b is turned off again, the step-up transformer 3 is turned off.
The electromagnetic energy stored in the power supply 1 is regenerated to the power supply 1 while being supplied to the high-voltage capacitor 7, and the operation shifts to a period in which the power MOSFETs of the two switching element groups 8a and 8b are simultaneously turned off.

Next, the power M of the switching element group 8a
When the OSFET is turned on, the secondary side circuit of the step-up transformer 3 has a high-voltage diode 6b, a high-voltage capacitor 7, one end 3d of the secondary winding of the step-up transformer 3, the other end 3e of the secondary winding, and a current flowing through the closed loop of the magnetron 5. The flow then supplies the magnetron 5 with electrical energy. When the power MOSFET of the switching element group 8a is turned off, the electromagnetic energy stored in the step-up transformer 3 is regenerated to the power supply 1 while being supplied to the magnetron 5. The above operation is repeated, and high frequency power is supplied to the magnetron 5.

At this time, the high-voltage capacitor 7 has circuit inductance, circuit capacitance, and circuit resistance in the main circuit, that is, the leakage inductance of the step-up transformer 3, the capacitance of the high-voltage capacitor 7 and the low-voltage capacitor 12, the switching element on-resistance, and the wiring resistance. Switching element group 8 that draws an arc of vibration determined by circuit resistance such as
b, the battery is charged with the same current waveform as the drain current waveform of the power MOSFET, and the magnetron 5 has the leakage inductance of the step-up transformer 3 and the high-voltage capacitor 7,
And a power MOSFET of the switching element group 8a that draws an arc of vibration determined by the circuit resistance such as the capacitance of the low-voltage capacitor 12, the switching element ON resistance, and the wiring resistance
The electric energy is supplied with a current waveform similar to the drain current waveform of FIG.

FIG. 2 shows a power MOSFET according to this embodiment.
FIG. 4 is a diagram illustrating an example of a waveform of a current flowing through T. The fact that the circuit output power can be improved will be described in detail with reference to FIG.
As described above, the current waveform oscillates at a natural frequency determined by the values of the leakage inductance of the step-up transformer 3, the capacitance of the high-voltage capacitor 7, the low-voltage capacitor 12, and the circuit resistance. The natural frequency F can be expressed by the following equation.

[0027]

(Equation 1)

Here, L: leakage inductance of the step-up transformer C: capacitance of the high-voltage capacitor R: circuit resistance n: turn ratio of the step-up transformer The power M is within a range from a half cycle to one cycle of this waveform.
In order to match the on-time of the OSFET, L,
When the values of C and R are set (1 / 2F ≦ TON ≦ 1 / F),
It can be seen that the drain current when the MOSFET is off is substantially zero, and zero current switching is possible, so that the occurrence of transition loss can be suppressed as much as possible and the switching loss can be reduced. In particular, the on-time is reduced by half.
By making them equal to the period (1 / 2F = TON), the current during the ON period of the power MOSFET (integral value during the ON period of the current waveform) becomes almost maximum, as shown in FIG. Can be maximized. If the natural frequency is too long or too short than the ON time, the current value during the ON period becomes small as shown in FIGS. 2B and 2C.

On the other hand, by utilizing the fact that the current waveform draws an arc of oscillation, the circuit constant is changed to
The current value is controlled by changing the natural frequency of the current waveform during the ON period. This effect can be obtained by changing any value of the circuit constants, that is, circuit capacitance, circuit inductance, and circuit resistance. Here, the largest effect can be expected as an example, and it can be easily realized. A method for switching the capacity of the low-voltage capacitor using a relay contact and its effect will be described.

FIG. 3 is a power supply circuit diagram according to a second embodiment of the present invention. The difference from the first embodiment is that three low voltage capacitors (12a, 12b, 12c) are inserted in parallel, and two of them are connected to the main circuit via relays 13a, 13b. The relays 13a, 1
A switching circuit 14 for performing ON / OFF control of 3b is added. For example, when the capacitances of these three low-voltage capacitors are set to 4.7 μF, 10 μF, and 22 μF, respectively, and the switching circuit 14 controls the on / off of the relay contacts, the input capacitor capacitance becomes ON / OFF of the relay contacts 13a and 13b. And when both 13a and 13b are on, 36.
7μF, 26.7μ when 13a is off and 13b is on
14.7 μF when F, 13a is on and 13b is off,
When both 13a and 13b are off, the voltage is 4.7 μF. As a result, the circuit capacitance in the main circuit changes, and the current waveform supplied to the magnetron changes as shown in FIGS.
It changes like the actually measured waveforms shown in FIGS. Here, the current average value can be obtained from the integrated value of the current waveform, and in each case, 146.3 mA, 93.6 mA, and 58.
9 mA and 31.1 mA. Since the power supplied to the magnetron is proportional to the average value of the current, the power supplied to the magnetron, that is, the power of the microwave oven can be easily controlled by switching the input capacitor capacity as described above, and the power in the ordinary PWM control can be obtained. Instead of changing the switching on time as in the control, FIG.
As shown in FIG. 7, since the period of the current waveform is changed without changing the switching-on time, power control can be performed without impairing the feature of performing zero-current switching. Therefore, there is no reduction in efficiency due to power control.

The same operation can be performed by changing the capacity of the high-voltage capacitor. However, it is difficult to switch over the contacts in the high-voltage circuit both in terms of cost and technology. Alternatively, the present embodiment is characterized in that it can be performed by changing it continuously.

[0032]

According to the present invention, since there is no need to use a DC / AC inverter, it is possible to provide a drive circuit for an inverter microwave oven which is inexpensive, has high power utilization efficiency, and has a high output. Furthermore, since the low-voltage DC power supply is directly converted to a high-frequency current, the largest and heaviest step-up transformer in the power supply circuit can be reduced in size and weight, and the drive circuit can be made more compact. .

Further, by controlling the ON time of the switching element and the oscillation period of the transient oscillation of the current waveform, a large circuit output can be obtained, and the switching loss due to zero current switching can be reduced.

Further, by controlling the capacitance of the low-voltage capacitor, output power control can be realized without impairing the effect of zero-current switching.

From the above, if the drive circuit of the present invention is used in a microwave oven, it can be used with power according to the content of cooking, such as normal cooking and food thawing, and it is highly efficient. A microwave oven driven by a low-voltage DC power supply, which could not be realized, is realized. The present invention can be widely used for magnetron driving.

[Brief description of the drawings]

FIG. 1 is a power supply circuit diagram according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a switching current waveform of the power MOSFET according to the first embodiment.

FIG. 3 is a power supply circuit diagram according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a waveform of a current supplied to a magnetron in a second embodiment.

FIG. 5 is a diagram showing a waveform of a current supplied to a magnetron in a second embodiment.

FIG. 6 is a diagram showing a waveform of a current supplied to a magnetron in a second embodiment.

FIG. 7 is a diagram showing a waveform of a current supplied to a magnetron in the second embodiment.

FIG. 8 is a circuit block diagram of a conventional inverter microwave oven driven by a commercial power supply.

FIG. 9 is a diagram illustrating a method of driving a conventional inverter microwave oven using a low-voltage DC power supply.

[Explanation of symbols]

1: DC power supply 2: Inverter circuit 3: Step-up transformer 4: Double voltage half-wave rectifier circuit 5: Magnetron 6: High voltage diode 7: High voltage capacitors 8a, 8b:
Switching element group 9a, 9b: Switching element drive circuit 10: Control circuit 11: Choke coil 12: Low voltage capacitor 13: Relay 14: Switching circuit

Continuation of front page (56) References JP-A-4-87185 (JP, A) JP-A-4-87184 (JP, A) JP-A-3-65061 (JP, A) JP-A-4-230989 (JP) JP-A-4-230988 (JP, A) JP-A-3-295189 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H05B 6/66 H02M 7/48 H02M 7/538

Claims (4)

(57) [Claims] 1. A drive circuit for an inverter microwave oven for directly converting a power supply to a high-frequency current, comprising one or more switching elements connected in parallel.
An inverter circuit comprising two switching element groups each comprising a switching element, and control means for performing on / off operations of the switching element; and a high-frequency alternating current supplied from the inverter circuit to a primary winding. Transformer for generating a high voltage to the transformer and a secondary winding connected to the transformer.
And a high-voltage diode and high-voltage capacitor
A voltage doubler rectifier circuit for supplying power to the netron; a low-voltage capacitor connected in parallel to the power supply; and a choke coil connected to one end of the low-voltage capacitor and connected in series to the power supply. A primary winding and the two switching element groups are connected to form a closed loop, and a connection point between the two switching element groups and a center tap of the primary winding of the step-up transformer are connected to both ends of the low-voltage capacitor. A drive circuit for an inverter microwave oven. 2. A control means in the inverter circuit , wherein the on-time is substantially half of the oscillation period with respect to the relationship between the oscillation period of the transient oscillation of the output current waveform of the inverter circuit and the on-time of the switching element. 2. The method according to claim 1, wherein the setting is made so as to fall within one cycle from one cycle.
The drive circuit of the inverter microwave oven described. A 3. A low-pressure condenser, which is connected in parallel with the power source 2 or more, inserted against circuit connected to the low pressure condenser, have a switching means for disconnecting
And, drive circuit according to claim 2, wherein the inverter microwave oven and performs the output power control without disturbing the timing of the zero-current switching and controlling said switching means by changing the low-pressure condenser capacity. 4. A choke coil connected in series to said power supply.
Circuit having two or more coils connected to the choke coil
Has switching means for insertion and removal from
And controlling said switching means to control said choke coil.
Of the zero current switching by changing the reactor of
Performing output power control without losing timing
3. A drive circuit for an inverter microwave oven according to claim 2, wherein
Road.